Detection of the position of a face and a center position and orientation of the face is important in order to recognize a person or to read facial expressions based on a face image.
For example, a method is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2004-310396 where the position of the face is detected in a high degree of precision without being influenced by movement of the person or by the background by processing an image taken of a person's face.
Moreover, in Unexamined Japanese Patent Application KOKAI Publication No. 2005-011097 there is disclosed technology for improving precision of determination of the presence or absence of a face based on values denoting the likelihood of each possible face center lines, where the possible face center line is set by using lateral edge image detected (both sides of the face) in a read-in image obtained by photographing a region where a driver can be present.